1. Field of the Invention
The present invention relates to a superconducting saddle-shaped dipole electromagnet mainly used to deflect charged particles (such as electrons and ions) and a process for producing the same.
2. Description of the Prior Art
Among electromagnets for deflecting charged particles, two types are known, namely a normal conducting type and a superconducting type. The former has a magnetic flux density of only about 1.5 teslas, and not only is it large in size and heavy in weight but its running cost is rather high. It is therefore customary to use a superconducting electromagnet having a higher magnetic flux density and requiring no energizing after the permanent current mode has been reached, for an apparatus requiring the deflection of charged particles having a large energy, such as an ion implantation apparatus having a tendency to increase the particle energy, or a syncrotron orbital radiation (SOR) apparatus.
A saddle-shaped dipole electromagnet as shown in FIG. 4 is usually used for deflection because it has an excellent uniformity in magnetic field and is provided with an effective countermeasure to the magnetic force. It has been customary to use Keystoned type cables having a large sectional area (approximately 10 mm.sup.2) and an inverted trapezoidal sectional shape, as a cable for saddle-shaped coils 2 wound on a beam duct 1 opposite to each other. Since the excitation current of this type of cable is as high as several thousand amperes, it requires a high-output power source. Also, the lead wire has such a large sectional area that a great amount of heat generated in the lead wire is liable to leak into the cryostat housing the coils. Consequently, evaporation of a refrigerant (generally liquid helium) for cooling down the coils housed in the cryostat increases leading to an increase in the running costs.
One solution to the above problems is to use a cable having a high aspect ratio with the same width but tis thickness decreased to one-third to one-tenth that of the conventional cable in place of a customarily used strand having a thickness of about 1 mm and a width of about 10 mm and made by stranding 20 or more element wires. But, in this case, the diameter of each element wire has to be reduced to one-third to one-fifth of that of a conventional one. This causes an increase in cost for wire drawing and makes the stranding work extremely difficult. Such a method is not a practical solution.
Using a small-sized cable to lower the current intensity will only increase the time required to wind a predetermined quantity of cable.
In winding a saddle-shaped coil, it is usually necessary to stack one layer upon another in the direction of thickness of the coil to form a several-layered structure so as to obtain the same quantity of winding. Since the winding is carried out from the inner layer toward the outer layer, as shown in FIG. 1 by arrows, it is impossible to form a continuous coil using a single cable with each layer interconnected to the adjacent layers. Instead, a cable has to be cut off every time one layer is wound up, and each layer has to be interconnected to its adjacent layers using crossovers at the last stage. This is very troublesome to do.